1. Field of the Invention
The present invention relates to a circuit calibrating method and a circuit calibrating system, and particularly relates to a circuit calibrating method and a circuit calibrating system which can control the number of active analog control signal generating units to reduce signal drift.
2. Description of the Prior Art
Some drift may occur during the manufacturing process for a circuit, such that all characteristics (ex. current, voltage, capacitance, and resistance) may have undesired values. Accordingly, a calibrating mechanism is needed.
FIG. 1 is a block diagram illustrating the prior art circuit calibrating system. As shown in FIG. 1, the circuit calibrating system 100 comprises a digital control code generating circuit 101 (hereinafter, a DCC generating circuit)and an analog control signal generating circuit 103 (hereinafter, an ACS generating circuit). The DCC generating circuit 101 is applied to generate a digital control code DCC such as 001, 010, 0110 . . . . The ACS generating circuit 103 comprises a plurality of analog control signal generating units (hereinafter, ACS generating units) U1, U2 . . . Un. The ACS generating units U1, U2 . . . Un can be some kinds of circuits, which comprise at least one transistor, and a capacitor. At least one of the ACS generating units U1, U2 . . . Un is activated corresponding to the digital control code DCC. The activated ACS generating units U1, U2 . . . Un provides analog control signals ACS to a target circuit 105 (i.e. the above mentioned circuit which needs to be calibrated). The target circuit 105 generates an output signal OS according to analog control signals ACS. The DCC generating circuit 101 receives the output signal OS and determines if the output signal OS has a desired value. If yes, recording the digital control code DCC being applied, such that it can be known that which control signal the target circuit 105 needs to generate the desired value. Or, the digital control code DCC is consecutively changed, until the output signal OS has the desired value.
FIG. 2 is a schematic diagram illustrating the ACS generating unit is activated by a digital control code, in prior art. Please note, for the convenience of understanding, the symbol of the ACS generating units correspond to a binary value. Take FIG. 2 for example, the first bit of the digital control code 001 is 1, such that the ACS generating unit with a symbol U1 is activated. In another example, the second bit of the digital control code 010 is 1, and the binary value thereof is 2, such that the ACS generating unit with a symbol U2 is activated. Following such rule, the first bit and the second bit of the digital control code 011 are 1, thus the ACS generating unit with symbols U1 and U2 are activated.
In this embodiment, the digital control code is a 3 bit code, and the ACS generating circuit comprises 7 ACS generating units with an ACS generating unit U1, 2 ACS generating units U2, and 4 ACS generating units U4. As depicted in FIG. 2, no ACS generating units are activated if the digital control code is 000. Also, if the digital control code is 001, only one corresponding ACS generating unit U1 is activated. If the digital control code is 010, two corresponding ACS generating units U2 are activated. If the digital control code is 011, corresponding ACS generating units U1, U2 are activated. If the digital control code is 100, 4 corresponding ACS generating units U4 are activated. In view of above-mentioned examples, the number for the activated ACS generating units is a binary code for the digital control code. However, such control method may cause larger signal drift.
For more detail, each ACS generating unit may have a signal draft amount which has a signal drift with a standard deviation δcell, thus the signal drift is more serious if more differently activated ACS generating units are activated for each time. For example, if the ACS signal generating units are respectively activated by the digital control code 011 and 010, only one differently activated ACS generating unit U1 exists, thus the analog control signals ACS generated due to the digital control code 011 and 010 may exist a signal draft amount which has a signal drift with a standard deviation δcell. Following the same rule, if the ACS signal generating units are respectively activated by the digital control code 001 and 010, three differently activated ACS generating units U1, U2 exist, thus the analog control signals ACS generated due to the digital control code 001 and 010 may exist a signal draft amount which has a signal drift with a standard deviation √{square root over (3)}δcell. Following the same rule, if the ACS signal generating units are respectively activated by the digital control code 011 and 100, seven differently activated ACS generating units exist (all U1, U2 and U4), thus the analog control signals ACS generated due to the digital control code 011 and 100 may exist a signal draft amount which has an signal drift with a standard deviation √{square root over (7)}δcell.
FIG. 3 is a schematic diagram illustrating a difference between the ideal output and the actual output for the ACS generating circuit, in prior art. As depicted in FIG. 3, some error may exist between the ideal output of the ACS generating circuit (i.e. the analog control signal ACS) and the actual output of the ACS generating circuit. However, such error may suddenly raises up for a specific stage of digital control code. The reason has been described in above-mentioned description, many different ACS generating units are activated for the specific stage of digital control code and a neighboring stage of digital control code for the specific stage of digital control code (ex. 011 and 100).
FIG. 4 is a schematic diagram illustrating how to activate the ACS generating unit via the digital control code and the decoder, in prior art. As depicted in FIG. 4, different stages of digital control codes activate ACS generating units in sequence, that is, the activated ACS generating unit does not correspond to particular bits. In such case, the number of differently activated ACS generating units for the digital control codes 011 and 010 can be reduced to 1. However, such method needs a decoder, which always occupies a large region of the circuit, to convert the original digital control codes to control codes which can activate ACS generating units in sequence.